Process for crosslinking olefin polymers

ABSTRACT

A PROCESS FOR CROSSLINKING AN OLEFIN POLYMER OR COPOLYMER CAPABLE OF BEING MOLDED BY HEATING THE OLEFIN POLYMER OR COPOLYMER IN THE PRESENCE OF A FREE RADICAL CATALYST AND A CROSSLINKING ADJUVANT, CHARACTERIZED IN THAT THE CROSSLINKING ADJUVANT IS TRIALLYL TRIMELLITATE AND/OR TRIMETHALLYL TRIMELLITATE. THE USE OF THE ABOVE-MENTIONED CROSSLINKING ADJUVANT RESULTS IN SUCH ADVANTAGES THAT THE CROSSLINKING EFFICIENCY IS ENHANCED AND THE CROSSLINKED POLYMER OR COPOLYMER IS IMPROVED IN HEAT RESISTANCE, CHEMICAL RESISTANCE, MECHANICAL STRENGTH AND ELECTRICAL PROPERTIES,

United States Patent 3,806,555 PROCESS FOR CROSSLINKING OLEFIN POLYMERS Joji Nagaoka, Tokyo, Nobuaki Minamii and Akio Henmi, Kawagoe, and Teruo Uchida, Omiya, Japan, assignors to Wako Pure Chemical Industries, Ltd.,' Osaka, Japan No Drawing. Filed Mar. 22, 1972, Ser. No. 236,974

Int. Cl. C08f 1/60, /00, 27/00 U.S. Cl. 260-878 R 13 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for chemically crosslinking polymers or copolymers of olefins (hereinafter referred to as olefin polymers). More particularly, the present invention pertains to a process for crosslinking olefin polymers by heating the polymers in the presence of a free radical catalyst and, as a cross-linking adjuvant, triallyl trimellitate (hereinfter abbreviated to AM) and/or trimethallyl trimellitate (hereinafter abbreviated to MM). The use of the above-mentioned crosslinking adjuvant results in such advantages that the crosslinking efficiency is enhanced and the cross-linked polymers are improved in various characteristics, particularly in heat resistance, chemical resistance, mechanical strength and electrical properties.

Olefin polymers are chemically stable, and cross-linking reaction products thereof have various characteristics. Accordingly, the olefin polymers have widely been used in industry. Because of their being chemically stable, however, the olefin polymers cannot be crosslinked by adoption of a crosslinking agent or a crosslinking process which is conventionally used in the case of unsaturated hydrocarbon rubbers. Further, for economical and other reasons, it is more practical to crosslink the olefin polymers by a chemical process than according to an irradiation process, which is a physical process. Particularly preferable is a crosslinking process using a free radical catalyst such as an organic peroxide or an azo compound. In practice, however, the crosslinking with an organic peroxide alone, for example, is low in crosslinking efflciency and cannot be expected to result in an improvement in various characteristics.

Moreover, in the case of olefin polymers containing such units as propylene, vinyl chloride or the like, such a problem is brought about that the polymers are considerably cut or modified during the crosslinking step, with the result that the crosslinking yield is lowered and no desired characteristics can be attained. With an aim to solve the above-mentioned problem, various blending agents have been studied, and the use of sulfur, quinone compounds and polyfunctional monomers as crosslinking adjuvants has been proposed. However, there has not been provided any crosslinking adjuvant capable of sufficiently settling the problem concerning the crosslinking of olefin polymers.

As a result of extensive research, the present inventors have found crosslinking adjuvants capable of sufiiciently imparting to the olefin polymers various characteristics necessary for the uses thereof over a wide field, thereby accomplishing a useful crosslinking process which can give an excellent crosslinking efficiency and which can be easily and economically practiced on a commercial scale.

In accordance with the present invention, there is provided a process for crosslinking an olefin polymer capable of being molded which comprises heating the olefin polymer in the presence of a free radical catalyst and, as a crosslinking adjuvant, triallyl trimellitate or trimethallyl trimellitate or a mixture of the two.

The crosslinking adjuvant used in the present process is AM or MM or a mixture of the two. The said AM or MM may be prepared on a commercial scale by reacting trimellitic acid or an anhydride thereof with allyl or methallyl alcohol or with an allyl or methallyl halide.

Although AM is commercially available, MM is a novel compound, which has been discovered by the present inventors, and which may preferably be prepared by the following method: Trimellitic anhydride and a slight excess of methallyl chloride or bromide are healed at about refluxing temperature in the presence of a base in a suitable solvent, and when the reaction is finished, the organic liquid layer is separated or extracted and then washed with water, after which the product is subjected to a conventional purification procedure to obtain MM having a boiling point of 204-6 C./2.5 mm. Hg. As the base, there may be used sodium hydroxide, potassium hydroxide, sodium carbonate, potassium bicarbonate, ammonia, pyridine, dimethylamine, ethylarnine or diethylaniline. The reaction is preferably efiected at a pH of 9 or less.

The amount of the crosslinking adjuvant added to the olefin polymer is not particularly limited, and is variable within a wide range according to various charactertistics, particularly heat resistance, chemical resistance, mechanical strengths and electrical properties, of the resulting crosslinked product. However, it is preferable to use the resulting crosslinked product. However, it is preferable to use the crosslinking adjuvant in an amount of 0.05 to 50 parts by weight per parts by weight of the olefin polymer. The addition of the crosslinking adjuvant can result in an increase of the crosslinking efiiciency obtained by adding the free-radical catalyst alone. The larger the amount of the crosslinking adjuvant added in a certain range, the higher the crosslinking etficiency becomes. Moreover, an increased amount of the crosslinking adjuvant does not result in such a degradation of characteristics as caused by increasing the amount of the free radical catalyst. Thus, the amount of the crosslinking adjuvant added is not irrelevant to the amount of the free radical catalyst or to the crosslinking efiiciency or the characteristics of the resulting crosslinked molded article.

Olefin polymers usable in the present invention are any polymers or copolymers containing, as monomers or comonomers, organic compounds having terminal unsaturated group represented by the formula C=CH EX- amples of the olefin polymers include polyolefins, such as polyethylene, polypropylene, etc.; polyvinyl chloride, various chlorinated polyolefins, such as chlorinated polyethylene, etc.; copolymers of olefins, such as ethylene-propylene copolymers, ethylene-butylene copolymers, etc.; copolymers of an olefin and other terminally unsaturated compound, such as ethylene-vinyl acetate copolymer, ethylenevinyl chloride copolymer, propylene-vinyl chloride copolymer etc.; and ethylene-propylene-diene terpolymers, such as ethylene propylene dicyclopentadiene terpolymer, ethylene propylene cyclooctadiene terpolymer, ethylene-propylene-hexadiene terpolymer, ethylene-propylene ethyltridecadiene terpolymer, ethylene-propylene-ethylidenenorbornene terpolymer, ethylene propylene methylidenenorbornene terpolymer, etc., though these are not limitative. The olefin polymers used in the invention are 5 such as can be molded.

As the free radical catalyst, any compound capable of liberating a free radical at the crosslinking temperature may be used. Generally, an organic peroxide or an azo compound is preferable. Examples of such free radical catatlyst include dicumyl peroxide, 1,3-bis(u-t-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, 2,5 -dimethyl-2,5-bis t-butylperoxy) -hexyne (3 di-t-butyl peroxide,

t-butyl perbenzoate,

tetrachloro-t-butyl peroxide, di(a-phenylethyl) peroxide, di(a-p-isopropylcumyl) peroxide, a-cumyl-u-p-t-butylcumyl peroxide, u-cumyl-a-p-xylyl peroxide, t-butyl-a-cumyl peroxide, t-butyl-triphenylmethyl peroxide, di-zx-p-cymyl peroxide,

dibenzyl peroxide, bis(a,a-dimethylnaphthylmethyl) peroxide, benzyl ot-methylbenzyl) peroxide, benzyl(a-methyl-p-methylbenzyl) peroxide, benzyl(a-methyl-p-isopropylbenzyl) peroxide, benzoyl peroxide,

acetyl peroxide,

lauroyl peroxide,

ascaridol,

2,2-azobis-isobutyronitrile,

2,2'-azobis (2,4-dimethyl valeronitrile) 1,1-azobis(cyclohexane-l-carbonitrile), phenylazo-2,4-dimethyl valeronitrile, phenylazo-Z,4-dimethyl-4-methoxy valeronitrile, 2-cyano-2-propylazo formamide, 2,2'-azoisobutane, 1,1'-azobis(2-phenylethane) 1,1-azobis(3-phenylpropane), etc.

The amount of the free radical catalyst added is not particularly limited, and may suitably be varied depending on the properties of the polymer to be crosslinked. However, it is preferable that the catalyst is used in an amount of 0.05 to parts by weight per 100 parts by weight of the olefin polymer. In case an azo compound as the free radical catalyst is used in a large amount, there is brought about the disadvantage that bubbles are incorporated into the resulting crosslinked and molded article, but no such disadvantage is brought about at all when the amount of the azo compound is less than 6 parts by weight.

The above-mentioned components are blended together according to a known procedure, and the resulting blend is transferred to a mold or the like and is treated in a hot lyst to form a free radical. The crosslinking time is markedly shortened by use of AM or MM, and can be varied depending upon the crosslinking temperatures, though it ranges preferably from 10 sec. to 60 min., particularly preferably from 1 to 5 min. This is another advantage of the present invention.

The present invention is further explained in detail below with reference to examples, which are only by way of illustration and not by way of limitation. In the examples, the preparation of each crosslinked resin and the measurement in physical properties thereof were carried out in the following manner:

The mixing of the olefin polymer with other components was carried out by means of a two-roll mill maintained at a definite temperature, and the resulting mixture in the form of a sheet was placed in a metal mold and then heated at a definite temperature for a definite period of time by use of a hot press of about 50 kg./cm. to prepare a crosslinked sample. The measurement in tensile strength of the sample was conducted at 20 C. by use of an Instron tester. The gel percent of the sample was measured in such a manner that the sample was treated for 10 hours in refluxed toluene (provided that in case polyvinyl chloride had been used, the sample was treated for 4 hours in dimethylformamide at C.), the insoluble portion of the sample was dried and then weighed, and the gel percent was calculated according to the following equation:

Weight of insoluble portion of sample Total Weight of sample Gel ercent:

In the examples, all the numerals and percentages showing the amounts are parts by Weight per 100 parts of the olefin polymer and percentages by weight, unless otherwise specified.

Reference example EXAMPLE 1 Blend.

Components:

Low density polyethylene (density: 0.921, M.I.:

1.5) (Mirason-9 trade name of Mitsui Polyehemical, .Tapan3 Stearie acid Dieumyl peroxide Each of the above-mentioned blends A, B and C was Xtreated at a crosslinking temperature of C. As the result, the relation between the heating time for crosslinking and the tensile strength, elongation and gel percent of the resulting crosslinked resin was as set forth in the following table:

Blend A Blend B Blend 0 Heating time (minutes) 0 5 10 15 30 40 0 5 10 15 30 40 0 5 10 15 30 40 Tensile strength (kg./cm. 127 171 190 201 194 190 126 175 192 215 210 195 126 123 158 160 172 Elongation (percent) 563 637 640 638 640 630 550 653 651 655 642 586 640 623 619 625 630 635 Gel (percent 43. 0 82. 7 85. 0 84. 8 84. 5 50. 3 85. 1 93. 2 89. 0 87. 0 Dissolved 40. 0 43. 45. 7 60. 1

plasticizers and stabilizers.

The crosslinking'temperature is above the thermal decomposition temperature of the free radical catalyst used and is selected from temperatures suflicient for the cata- As is clear from the above table, each blend was quick in gelation, i.e. shortened in crosslinking time, and the desired tensile strength and elongation were attained at earlier stages.

EXAMPLE 2 EXAMPLE 8 The blend A mentioned in Example 1 was treated 318mm K L at a crosslinking temperature of 190 C. to obtain the Component following results. Ethylene-vinyl acetate resin having a vinyl acetate con- 5 tent of 19% (EVAFLEX-P-1905, trade name of Mitsui Polyehemical, Japan) 100 100 2 2 5 S 11 Heating time (mintues) 1 2 3 5 H we 9 3 i i as; tit 55 a 52 t' t adlilrlt j i 9 ,0 90,5 91,0 88.5 86,1 85.0 10 The above menfloned blends K and L f'f l treated for minutes under the same c0nd1t1ons as in Comparing the above results with those in Example 1, Example 1 to obtain the results as set forth 1n the followit is found that when the crosslinking temperature 1s mg tab higher, the heating time can be much more shortened.

EXAMPLE 3 15 Blend L a I I I u The blend D, WhlCh was identical with the blend A gfg jggggg tggl gfiggfif Z 315 in Example 1 except that 1 part of MM obtained 1n sho p rdnessz 95 83 the reference example was used in place of the AM, g::::::::::II: 83 51 was thermally treated in the same manner as in Example 20 Elas2tgly (percent): 35 27 1 at a crosslinking temperature of 165 C. to obtain 755Cjjjjjjjjjjjjjjjjjjjj: 56 35 the results as set forth in the following table:

Blend D Blend 0 Heating time (minutes) 0 5 10 15 40 0 5 10 16 30 40 128 184 208 199 197 185 126 154 158 162 175 159 Elongation (percent)... 565 605 600 592 575 565 640 570 575 570 578 567 Gel (percent) 70.8 91.0 91.1 90.4 85.1 27.7 42.5 53.5 66.5 55.8

From Examples 1 and 3, it is understood that by 30 EXAMPLE 0 addition of AM or MM, the crosslinking time is shortened M N and the resulting crosslinked resin has an increased 0 ts omponen tensile strength and an increased gel content. golmthvl'anusameasm Example 1) 10(1) eanc 8.01 Dicumyl peroxide 1 2 EXAMPLES 4-6 MM (obtained in the Reference Example) 2 2 The blends E, F and G comprising 100 of polyvinyl chloride having an average degree of polymerization of 1.070 and a bulk density of 0.50 (TK-lOOO, trade name of Shinetsu Kagaku, Japan), 50 of dioctyl phthalate 40 (DOP), 5 of dibutyltin dilaurate (DBL), 2 of oz,ot-biS(t butylperoxy)-m-isopropylbenzene (catalyst) and 5, 15

The above-mentioned blends M and N were thermally treated at a crossinking temperatures of 165 C. to obtain the results as set forth in the following table, in which are also shown the results obtained by thermally treating at said temperature the blends A and C used in Example 1.

and 30, respectively, of AM were thermally treated for Blend A C M N 20 minutes under the same conditions as in Example 3 5 'lljfnsileflstrerzgth (kg),/ 2) g2 g 23 onga on ercent 8 to obtain crosslmked resins having the following tensile Gel (percent? 9M 4Z5 8&5 9L0 strength and gel percent.

Tensile Elon- Gel Example Catastrength gation (pernumber Blend PVC DOP DBL lyst AM (kg/cm!) (percent cent) In contrast thereto, a crosslinked resin prepared in the same manner as above, except that no AM was added, had a tensile strength lower than that of the starting polyvinyl chloride.

The above-mentioned blends H, I and I were thermally treated for 20 minutes under the same crosslinking conditions as in Example 3 to obtain the results as set forth in the following table:

Rlen (l H I I Tensile strength (kgJcm 82 57 Elongation (percent)--. 657 670 595 As mentioned above, the use of AM and/or MM as a crosslinking adjuvant in crosslinking olefin polymers with a free radical catalyst results in the enhancement of the crosslinking efliciency, whereby various properties of the polymers can be improved. In particular, the gel percent is increased, whereby the heat distortion temperature is elevated and the distortion due to a load becomes remarkably small. For instance, when polyethylene, dicumyl peroxide, stearic acid and AM were mixed in the following proportions and heated at 165 C. for 14 min., the distortion and the gel percent of the resulting crosslinked polymer were as shown in the following table (according to HS 3606 and 3005):

Parts by weight Percent Polystearic Distor- Distorethylene D C P acid AM tion 1 tion 2 Gel NOTES:

DCP refers to dicumyl peroxide. Distortion 1 is under a load of 2 kg. Distortion 1 is under a load of 4 kg.

As is clear from the above table, the distortions under a load of 2 kg. and 4 kg. in Run No. 2 are A and A, respectively, of the distortions in Run No. 1 where no AM was added. Further, as seen from Run Nos. 1 and 3, the distortion under a load of 4 kg. when 2 parts by weight of dicumyl peroxide is used without using AM is reduced to /2 by substituting 1 part by weight of AM for 1 part by weight of the dicumyl peroxide. Moreover, the dielectric loss tangent (tan 6) and dielectric constant of polyethylene crosslinked by means of 1 part by weight of AM and 2 parts by weight of dicumyl peroxide per 100 parts by weight of polyethylene is as very low as 0.5X10- and 2.1 at 3 mHz., respectively. The dielectric constant was not varied even when the amount of AM was varied. In addition, -AM serves as a plasticizer in blending prior to crosslinking, and hence, a conventional plasticizer, which generally results in a degradation of electrical properties of polymers, is made unnecessary.

What is claimed is:

1. A crosslinked polymer obtained by heating at least one polymer selected from the group consisting of polyolefins, copolymers of olefins, ethylene-vinyl acetate copolymer, and ethylene-propylene-diene terpolymers together with triallyl trimellitate, trimethallyl trimellitate or a mixture of the two in the presence of a free radical catalyst.

2. A crosslinked polymer according to claim 1, wherein the polymer is selected from the group consisting of polyethylene and polypropylene.

3. A crosslinked polymer according to claim 1, wherein the polymer is selected from the group consisting of ethylene-propylene copolymer and ethylene-butylene copolymer.

4. A crosslinked polymer according to claim 1, wherein the polymer is selected from the group consisting of ethylene-propylene-dicyclopentadiene terpolymer, ethylene-propylene-cyclooctadiene terpolymer, ethylene-propylene-hexadiene terpolymer, ethylene propylene ethyltridecadiene terpolymer, ethylene-propylene-ethylidenenorbornene terpolymer, and ethylene-propylene-methylidenenorbornene terpolymer.

5. A crosslinked polymer according to claim 1, wherein the free radical catalyst is an organic peroxide or an azo compound.

6. A crosslinked polymer according to claim 5, wherein the organic peroxide is dicumyl peroxide, 1,3-biS-(at-butyl-peroxyisopropyl)benzene, 2,5 dimethyl-2,5-bis (tbutylperoxy)hexane, 2,5-dimethyl-2,5-bis(t-butylperoxy) hexyne (3), di-t-butyl peroxide, t-butyl perbenzoate, tetrachloro-t-butyl peroxide, di(a-phenylethyl)peroxide, di- (a-p-isopropylcumyl)peroxide, a-cumyl-wp-t-butylcumyl peroxide, a-cumyl-u-p-xylyl peroxide, t-butyl-a-cumyl peroxide, t-butyl-triphenylmethyl peroxide, di-a-p-cymyl peroxide, dibenzyl peroxide, bis(a,a-dimethylnaphthylmethyl)peroxide, benzyl(ot-methylbenzyl) peroxide, benzyl(amethyl-p-methylbenzyl) peroxide, benZyl(a-methyl-p-isopropylbenzyl) peroxide, benzoyl peroxide, acetyl peroxide, lauroyl peroxide, or ascaridol.

7. A crosslinked polymer according to claim 5, wherein the azo compound is 2,2-azobisisobutyronitrile, 2,2- azobis'(2,4-dimethyl valeronitrile) or 1,1'-azobis(cyclohexane-l-carbonitrile), phenylazo-2,4-dimethyl valeronitrile, phenylazo-2,4-dimethyl-4-methoxy valeronitrile, 2- cyano-2-propylazoformamide, 2,2-azobisisobutane, 1,1- azobis- (Z-phenylethane) or 1,1'-azobis(3-phenylpropane) 8. A crosslinked polymer according to claim 1, wherein the amount of the free radical catalyst is 0.05 to 10 parts by weight per 100 parts by weight of the polymer or copolymer.

9. A crosslinked polymer according to claim 1, wherein the amount of the crosslinking adjuvant is 0.05 to 50 parts by weight per 100 parts by weight of the polymer.

10. A crosslinked polymer according to claim 1, wherein the heating temperature is higher than the decomposition temperature of the free radical catalyst and is a temperature high enough to form a free radical.

11. A crosslinked polymer according to claim 1, wherein the heating time is 1 to 5 minutes.

12. A crosslinked polymer according to claim 1, wherein the polymer contains any of antioxidants, fillers, pigments, plasticizers and stabilizers.

13. A crosslinked polymer according to claim 1, wherein the heating time is 10 seconds to minutes.

References Cited UNITED STATES PATENTS 3,261,888 7/1966 Cornell et al. 260-878 R 3,312,757 4/1967 McRitchie 260-878 R FOREIGN PATENTS 905,711 9/1962 Great Britain 260-884 OTHER REFERENCES Murphy, Jr. et al.: Def. Pub. Search Copy of SN. 135,207, filed Apr. 19, 1971, published in 900 0.6. 1231 on July 25, 1972, Defensive Publication No. T900,019 (260/878 R).

JAMES SEIDLECK, Primary Examiner A. HOLLER, Assistant Examiner U.S. C1. X.R.

260-78.40, 80.78, 87.3, 88.2 S, 93.7, 94.9 GA 

